The present invention relates to an image forming apparatus used, for example, in an electrophotographic copying machine.
As this type of image forming apparatus, an apparatus as shown, for example, in FIG. 11 is known. That is, reference numeral 101 denotes a drum-like photosensitive body which is rotatable as an image carrier. Around the circumference of the photo-sensitive drum 101, a charging brush roller 102, a developing device 104, a transfer roller 105, a cleaning device 106 and a charge eliminator 107 are arranged along a rotation direction (indicated by an arrow) of the photosensitive drum.
At a time of forming an image, the surface of the photosensitive drum is uniformly charged by the rotation of the charging brush roller 102. Image information light 109 is radiated onto the charged surface of the photosensitive drum 101 by means of a light exposure device, not shown, to form an electro-static latent image there. The electrostatic latent image is sent by the rotation of the photosensitive drum 101 to a position where it faces the developing device 104 and is then developed by being supplied with a toner from the developing device 104. In this way, a toner image is created there. The toner image is electrostatically transferred by the transfer roller 105 to a conveying sheet P. The sheet having the transferred toner image is heated/pressed by a fixing unit, not shown, to allow the image to be fixed to the sheet and a given image to be formed there.
After the toner image has been transferred to the sheet, residual toner on the photosensitive drum 101 is cleaned off by the cleaning device 106. After the cleaning step, the surface of the photosensitive drum 1 is discharged by the charge eliminator 107.
In recent years, there has been a tendency toward more compact image forming apparatuses and not only the photosensitive drum 101 but also the surrounding mechanisms (developing device 104, cleaning device 106, charge eliminator, etc.) have been made more compact. Further, the distance between the respective mechanisms has been reduced and there is a growing demand that the charging section also be made more compact. If the charging section is made smaller, then there arises charging failure.
If, for example, charging is made by the charging brush roller 102 and, in this case, the roller diameter is made smaller, then the surface area becomes smaller and the brushing amount involved is decreased. As a result, less contact is involved between the brush and the photosensitive drum 101, so that there arises a charging failure. In order to maintain a better charging level, it is essential to secure an adequate roller diameter.
In order to secure an adequate diameter of the charging brush roller 102, however, it becomes necessary that the distance L between the center of the charging brush roller 102 and the image information light 109 be made greater. That is, unless the distance L is made more than one half the diameter of the charging brush roller 102, the outer peripheral portion of the brush 102a blocks out the image information light 109, so that it is not possible to give exposure light.
In the case where a charging roller 102 of a greater outer diameter is used, the distance L becomes greater and there is the inconvenience that it is not possible to provide a compact apparatus.